1. Field of the Invention
The present invention relates generally to the field of ferrofluidic seals. Particularly, the present invention relates to a multi-stage ferrofluidic seal assembly. More particularly, the present invention relates to a multi-stage ferrofluidic seal assembly that incorporates more than one type of ferrofluid.
2. Description of the Prior Art
Ferrofluidic rotary seals have been widely used in vacuum applications over the past 20 years and are commonly employed to transmit rotary motion into a process chamber under high vacuum. The basic structure of the seal comprises magnets, a rotary shaft, magnetic pole pieces or poles, and a housing. The magnets, the poles, and the shaft form magnetic circuits with air gaps between the poles and the shaft. A ferrofluid is attracted to the air gap and forms the dynamic sealing between the poles and the rotary shaft. The sealing between stationary parts, such as that between a pole and its housing, is usually provided by a rubber O-ring at the radial interface.
Seals with this structure have been effective for a wide variety of applications, such as semiconductor manufacturing, optical coating, and rotary gas union, etc. However, in recent years, there are an increased number of applications that have increasingly demanding performance requirements. These requirements include, but are not limited to, ultra-high vacuum (UHV) capability, high speed, low starting/running torque, low heat generation, long life, chemical stability, and chemical compatibility.
Frequently, these requirements are contradictory to one another. For example, a ferrofluid suitable for UHV application typically has a low evaporation rate and high viscosity. It tends to have a high starting/running torque that makes it unsuitable for high-speed applications.
The properties of ferrofluids control the application for which a particular ferrofluid is suited. Ferrofluids are magnetically responsive colloidal liquids. The main constituents are nano-sized magnetic particles, one or more surfactants and a base carrier. Each particle is a permanent magnet of substantially spherical shape with a diameter of about ten nanometers. The particles are coated with a surfactant that keeps the particles separate from each other and prevents them from coalescing under the attractive Van der Waals and magnetic forces. The liquid medium is which the particles are suspended is often referred to as the carrier. In a seal grade ferrofluid, the constituents typically have the following volume fractions: magnetic particles (8%), surfactant (16%) and carrier (76%).
The magnetic properties of a ferrofluid are determined by the volume fraction of the solid component. The greater the solid amount (which is the same as the magnetization of ferrofluid), the higher the pressure holding capacity of the sealing device. By far the dominant component of the ferrofluid is the carrier. It determines the physical characteristics of a ferrofluid such as viscosity, vapor pressure, operating temperature range, volatility, thermal conductivity, and environmental compatibility, and ultimate service life and power consumption of the seal. Thus, a proper selection of ferrofluid is crucial to the successful performance of a magnetic fluid rotary seal.
Due to the strong influence of the choice of carrier on seal operation, ferrofluids are designated by their carrier type. For example, ferrofluids based on any oil in the class of perfluoropolyethers (fluorocarbons), hydrocarbons, esters, polyphenyl ethers, and silicones are called fluorocarbon, hydrocarbon, ester, polyphenyl ether, and silicone-based ferrofluids, respectively. They may all have the same colloidal sized magnetic particles but would require different surfactants with matching molecular structure of the carrier.
Current sealing applications typically utilize one of the three families of ferrofluids namely fluorocarbons, hydrocarbons or esters depending upon the environments to be sealed. All of the stages in the multi-stage seal are charged with only one type of ferrofluid. This often leads to compromising the seal performance either in regards to power consumption, environmental compatibility or life of the seal.
The ferrofluid in rotary shaft seals forms distinct O-rings with intervening air cavities. There is a change in pressure as the chamber is evacuated and back filled with process gases. For a detailed discussion see Magnetic Fluids and Applications Handbook, Begell House, New York, which is incorporated herein by reference. Volatility of a ferrofluid like any other liquid depends on ambient pressure. Under high vacuum (approximately 10−7 torr) a typical seal grade ferrofluid evaporates roughly twenty times faster than at atmospheric pressure (approximately 760 torr). At one torr, the volatility is about 4 times higher than at 760 torr. In general, ferrofluid life is reduced both by evaporation and by any chemical reaction with the gaseous medium. Ferrofluid stages on the vacuum side degrade much faster than those on the atmospheric side. This is due in part to exposure to high vacuum and/or in part to hazardous gases (when present), which is not experienced by the stages on the atmospheric side.
Rotary shaft seals utilizing magnetic fluids are typically designed with a sufficient safety margin in pressure holding capacity so that several of the atmospheric side stages act as reserves. They experience conditions of only ambient air consisting of mostly nitrogen (approximately 80%) and oxygen (approximately 20%) independent of the conditions that exist on the process side. Like any other liquid O-ring located inside of the seal, the first vacuum-side ferrofluid O-ring (or stage) has two free surfaces. The surface exposed to the process chamber evaporates rapidly due to prevailing high vacuum conditions. On the other hand, the second surface of the same O-ring evaporates more slowly due to the pressure of the interstage region. This pressure typically ranges from 2 to 5 psi, depending upon the seal design.
Similarly, the second vacuum-side ferrofluid O-ring also experiences different evaporation rates at its two surfaces due to different gaseous pressures in adjoining cavities. However, the evaporation of the second stage ferrofluid is much less than the first stage. The second stage is expected to last longer than the first stage. The third stage has an even longer life than the second stage. The stages on the atmospheric side have the least volatility and longest life because of the maximum pressure (760 torr or 14.7 psi) in adjoining cavities. Nonetheless, environments surrounding a ferrofluid are an additional mechanism by which the fluid seal may fail.
For instance, when a hydrocarbon-based ferrofluid is used in a seal, the atmosphere-side stages deteriorate faster (even when the volatility is low) than when an ester or a fluorocarbon-based ferrofluid is utilized. The presence of oxygen in air trapped in the interstage regions reacts with hydrocarbons and causes the fluid to congeal over time. Under high condensable humidity, esters are not stable and thus hydrocarbons and fluorocarbons are the preferred fluids. Overall, fluorocarbon-based ferrofluids are far superior to other classes of ferrofluids in regards to environmental compatibility, long service life and ultra low vapor pressure. They are durable under radiation, humidity, reactive gases, and high temperature. Typically, fluorocarbon-based ferrofluids have a life 16 to 73 times longer than other fluids. However, the disadvantage of the fluorocarbon-based ferrofluids is that their viscosity is high. This results in greater start-up and running torque for the seal, requiring large and expensive motors to operate the device. The high viscosity also increases seal temperature, which requires liquid cooling of the device and adds to the cost of the product.
U.S. Pat. No. 4,407,518 (1983, Moskowitz et al.) discloses a nonbursting multiple-stage ferrofluid seal and system. The system includes an annular permanent magnet and annular first and second pole pieces where one end of the pole pieces defines a single-stage ferrofluid seal under one end of one pole piece and a multiple-stage ferrofluid seal under the one end of the other pole piece with the surface of the shaft element with which the seal is employed. The first and second pole pieces and the magnet define therebetween an interstage volume between the single-stage ferrofluid seal and the multiple-stage ferrofluid seal. A conduit means extends into the stage volume and connects to a means to maintain a desired pressure in the stage volume. This device uses a single, common ferrofluid throughout the seal and uses a compensating pressure to maintain seal integrity.
U.S. Pat. No. 4,865,334 (1989, Raj et al.) discloses a long-life multi-stage ferrofluid seal incorporating a ferrofluid reservoir. The reservoir is located between the seal stages and contains a quantity of ferrofluid sufficient to replace ferrofluid in the seal stages, which is lost due to evaporation or contamination. This device also uses a single type of ferrofluid but uses a reservoir system to replenish failed seals.
U.S. Pat. No. 4,335,885 (1982, Hooshang Heshmat) discloses a plural fluid magnetic/centrifugal seal to provide a hermetically sealed space between a rotated shaft member and a close fitting spaced-apart stationary housing where the housing and the shaft are shaped to provide magnetic pole-like close clearance gap regions between their opposed surfaces. A high viscosity ferromagnetic fluid normally is disposed in the magnetic gap region with the rotating shaft member at rest and at low rotational speeds. A permanent magnet or electromagnet is provided which forms a closed magnetic circuit through the magnetic gap region with the high viscosity ferromagnetic fluid. A circumferentially arranged centrifugal seal forming region is radially disposed outward from the magnetic gap region and is located between the rotatable shaft and the stationary housing member. A low viscosity centrifugal sealing fluid is disposed in the centrifugal seal forming region and is centrifugally thrown outwardly during high speed rotation of the rotating shaft member to form a centrifugal hermetic seal between the rotating shaft member and the housing at high rotational speeds of the rotating member.
Although this device uses plural fluids, the centrifugal fluid is not a ferrofluid but is comprised of water, lubricating oil, or other low viscosity fluid which does not heat up at the higher speeds of the speed range over which the seal is designed to operate. Further, the area surrounding the vane that rides in the casing where the centrifugal seal is created is nonmagnetic.
Therefore, what is needed is a multi-stage, ferrofluid seal that is capable of operation under high vacuum environments. What is also needed is a multi-stage, ferrofluid seal that is capable of overcoming the high viscosity torque required with fluorocarbon-based ferrofluid without the need for large and expensive motors. What is further needed is a multi-stage ferrofluid seal that has relatively low heat generation. What is still needed is a multi-stage, ferrofluid seal that combines the environmental compatibility and long service life characteristics of fluorocarbon-based ferrofluid seals with the low torque and low heat generation characteristics of non-fluorocarbon-based ferrofluid seals.